Rotary fluid pumps and motors transport fluid between distinct pressure regions. Such pumps and motors are characterized by housings having low and high pressure fluid delivery ports. Fluid pressure knows no preferred direction and acts uniformly in all directions. High pressure fluid at and in the vicinity of the high pressure port of such a pump exerts a force against the pump or motor's rotary structure in the direction of the low pressure port which force tends to displace and/or flex the rotary structure toward the low pressure port resulting in aggravated bearing wear and failure, housing wear, and perhaps even failure of the housing.
The invention is particularly concerned with rotary gear pumps such as shown, for example, in U.S. Pat. No. 2,714,856. Gear pumps of the character to which the invention relates typically comprise a housing which has a low pressure fluid inlet port and a high pressure fluid outlet port, mating driving and driven gear impellers mounted in the housing between the inlet and the outlet ports by integral hubs projecting from opposite sides of the impellers supported within the housing by needle bearings and side or end plates mounted in the housing at opposite sides or ends of the impellers.
Gear pumps or motors as described above typically have a pumping capacity of up to 50 gallons per minute. Impellers for these gear pumps generally have widths in the range from 1/2 to 3 inches. Gear pumps capable of higher pumping capacity provide a quicker, stronger response to the load in a given hydraulic system and require more surface area and generally tend to have wider impellers. For a given pump pressure, a larger surface area results in a greater force being applied against the impellers and therefore a greater load on the bearings supporting the impeller structures.
The efficiency of gear pumps is dependent in part on the housing interior surface closely conforming to the profile of the adjacent impellers. The total clearance between the impellers and the interior surface is typically no more than 0.003 inches. Consequently, it is important that the impellers run true within the housing, to avoid interference with the adjacent housing interior.
The high pressure fluid exhausting from the pump outlet exerts a force against the mating impellers in the direction of the low pressure inlet. The size of this force is determined by the amount of pressure at the pump outlet and the impeller surface area against which this high pressure may act. The high pressure generated tends to deflect the impeller structures in the direction of the low pressure inlet. The needle bearings which support the impeller hubs are forced out of full surface contact or load and into edge contact with the hubs. Once the needle bearings loose full surface contact or load with the hubs, pump failure is inevitable. The needle bearings dig into the hubs along one of their edges to cause spalling on the adjacent hub surface. The spalling creates an aggravated wear condition resulting from the interaction of the chips of material displaced from the hub with the relatively rotating parts of the pump and the needle bearings.
The displacement of material from the hubs by the needle bearings destroys the close tolerance fit at the impeller supporting structure. Since the clearance between the impellers and the housing is relatively small, a small amount of spalling creates sufficient play so the impellers can be displaced against the interior surfaces of the housing. When the impellers do contact the interior surfaces of the housing, they gouge that surface. Consequently, the housing interior surface wears and leakage results. If the fluid pressure at the pump outlet is sufficiently great, it is actually possible for the displaced impellers to fracture the housing.
The problems of uneven pressure distribution in a gear pump can be further aggravated by the environment in which the pump is used. In coal mines gear pumps are frequently used in hydraulic systems. These gear pumps usually pump, and are lubricated by, an oil-water mixture known as fireproof oil. The oil-water mixture is employed to minimize fire hazards within the mine. Because oil is mixed with water, the lubrication quality of this mixture is less than unadulterated oil. Consequently, the load rating is reduced for needle bearings in pumps utilized in such an environment. The problem is further aggravated by the practical consequences of operating in a mine environment. Workers in a mine do not rigorously replenish the fireproof oil supply in the proper oil-to-water proportion. Initially, fire-proof oil contain 80% oil and 20% water. As the fireproof oil reservoir is depleted, it is a common practice to merely add more water to the reservoir. Consequently, the lubrication quality of the fire-proof oil diminishes and the life of the pump is shortened as the load rating on the needle bearings is further reduced.
Gear pumps lubricated with fire-proof oil in a mine environment undergo tremendous wear. Usually these pumps are not fit to rebuild and there is usually nothing worth salvaging upon teardown. If these pumps are run at 1500 psi, they may last for approximately 3 months. If these pumps run at 2000 psi, they may last for a month. With higher pressures their life expectancy is even less.